Steel sheet for hot stamped member and method of production of same

ABSTRACT

A steel sheet for obtaining a member which is excellent in fatigue characteristics equal to ordinary high strength steel sheet of the same strength even if applying the hot stamping process and a method of production of the same are provided. 
     Steel sheet for a hot stamped member which includes composition which contains, by mass %, C: 0.15 to 0.35%, Si: 0.01 to 1.0%, Mn: 0.3 to 2.3%, Al: 0.01 to 0.5%, and a balance of Fe and unavoidable impurities, and limit the impurities to P: 0.03% or less, S: 0.02% or less, and N: 0.1% or less, wherein that a standard error of Vicker&#39;s hardness at a position of 20 μm from the steel sheet surface in the sheet thickness direction is 20 or less. 
     This steel sheet is produced by a recrystallization-annealing step of a first stage of heating a cold rolled steel sheet, which is obtained by hot rolling steel containing the above composition and then cold rolling it, by an average heating rate of 8 to 25° C./sec from room temperature to 600 to 700° C., then a second stage of heating by an average heating rate of 1 to 7° C./sec to 720 to 820° C.

TECHNICAL FIELD

The present invention relates to steel sheet for a hot stamped memberwhich is suitable for the hot stamping method, one of the shapingmethods giving a high strength member, and a method of production of thesame.

BACKGROUND ART

In the field of automobiles, construction machinery, etc., vigorousefforts are being made to reduce weight by use of high strengthmaterials. For example, in automobiles, the amount of use of highstrength steel sheet has been steadily increasing for the purpose ofcancelling out the increase in vehicle weight accompanying theimprovements in impact safety and performance and furthermore improvingfuel efficiency to reduce the amount of emission of carbon dioxide.

In the trend toward expanded use of such high strength steel sheet, thebiggest problem, unavoidable when raising the strength of steel sheet,is the rise of the phenomenon called “degradation of the shapefixability”. This phenomenon is the general term for loss of ease ofobtaining a target shape due to the increase in the amount of springbackafter shaping accompanying higher strength. To solve this problem,working steps which were unnecessary with low strength materials(materials with shape fixabilities which are excellent or not a problem)(for example, restriking) have been performed or the product shapes havebeen changed.

As one method for dealing with this situation, the hot shaping methodcalled the “hot stamping method” has come under attention. This heats asteel sheet (worked material) to a predetermined temperature (generally,the temperature resulting in an austenite phase) to lower the strength(that is, facilitate shaping), then shapes it by a die of a lowertemperature than the worked material (for example room temperature) tothereby easily impart a shape and simultaneously utilize the temperaturebetween the two for rapid cooling heat treatment (quenching) so as tosecure the strength of the shaped product.

Several arts relating to steel sheet suitable for such a hot stampingmethod and method of shaping the same have been reported.

PLT 1 shows steel sheet obtained by controlling the amounts of elementswhich the steel sheet contains and the relationship among the amounts ofthe elements to predetermined ranges so as to give a member which isexcellent in impart characteristics and delayed fracture characteristicafter hot shaping (synonymous with hot stamping).

PLT 2, in the same way as the above, discloses a method comprisingmaking the amounts of elements which the steel sheet contains and therelationship among the amounts of the elements to predetermined rangesand heating before shaping the steel sheet in a nitriding atmosphere ora carburizing atmosphere so as to obtain a high strength part.

PLT 3 describes means for prescribing the composition and microstructureof steel sheet and limiting the heating conditions and shapingconditions so as to obtain hot pressed parts with a high productivity.

Recently, the hot stamping method has become widely recognized for itsusefulness. Members for which its application has been studied havebecome much more diverse. Among these, for example, there are parts,such as underbody parts of automobiles, where not only the strength ofthe parts, but also the fatigue characteristic is an important,necessary characteristic.

The fatigue characteristic of steel sheet is improved together with thestatic strength, so steel sheet (product) made high in strength by thehot stamping method also can be expected to exhibit a commensuratefatigue characteristic, if compared with steel sheet of the samestrength not using the hot stamping method (high strength steel sheetproduced by controlling the composition or method of production of thestrength steel sheet, below, called “ordinary high strength steelsheet”), it became clear that depending on the production conditions,the fatigue characteristics of the former were inferior to the latter.

Studied in detail, it was discovered that compared with the deviation inhardness of the surfacemost part of “ordinary high strength steelsheet”, the deviation in hardness of the surfacemost part of steel sheet(product) raised in strength by using the hot stamping method is larger.It was concluded that this deviation in hardness might be related to thefatigue characteristic.

The relationship between the deviation in hardness and the fatiguecharacteristic is not necessarily clear, but in a high strength memberwhich is produced by the hot stamping method (for example, a tensilestrength of 1500 MPa or more), the effect of the notch sensitivity onthe fatigue characteristic is extremely large, so it is guessed thatthis deviation in hardness might be an indicator comparable to theflatness of a surface layer.

Therefore, the inventors studied the art for reducing as much aspossible the deviation in hardness after hot stamping and as a resultdiscovered that the deviation in surface layer hardness of the steelsheet before hot stamping has an impact. No literature has been foundwhich studies steel sheet for hot stamping use from such a perspective.

PLT 1 discusses steel sheet for hot shaping use where all of Ni, Cu, andSn are essential, wherein the impact characteristics and the delayedfracture characteristic are improved, but does not allude to the fatiguecharacteristic or the deviation in surface layer hardness before hotstamping.

PLT 2 relates to the art of heating in a carburizing atmosphere so as toraise the strength of a shaped part, but does not allude to the fatiguecharacteristic or the deviation in surface layer hardness before hotstamping. Heating in a carburizing atmosphere is essential. Comparedwith heating in the air, the production costs rise. Further, when usingcarbon monoxide as the source of carbon, there is a concern thattremendous costs would be required for securing the safety ofoperations. It is believed that this art is not easily workable.

PLT 3 also does not allude to the fatigue characteristic and thedeviation in surface layer hardness before hot stamping.

As opposed to this, as art for obtaining steel sheet for hot stampinguse which has a fatigue characteristic of the same extent as “ordinaryhigh strength steel sheet”, there is PLT 4. Further, while as artinherent to the case of use of steel sheet which has been galvanized,PLT 5 is known as art for improving the fatigue characteristic of amember which is produced by the hot stamping method.

PLT 4 discloses to make fine particles which contain Ce oxides disperseslight inward from the steel sheet surface so as to improve the fatiguecharacteristic after hot stamping, but advanced steelmaking art isrequired, so there is the problem that even a person skilled in the artwould not necessarily find it easy to work it.

The art of PLT 5 relates to facilities for hot stamping technology.There is the problem that without new capital investment, even a personskilled in the art could not enjoy its benefits.

In this way, steel sheet for hot stamping use for obtaining steel sheet(product) made high in strength by hot stamping, which enables fatiguecharacteristics of the same extent as “ordinary high strength steelsheet” of the same strength to be secured relatively easily, has beensought, but no art which solves this problem has been found.

CITATIONS LIST Patent Literature

PLT 1: Japanese Patent Publication No. 2005-139485A

PLT 2: Japanese Patent Publication No. 2005-200670A

PLT 3: Japanese Patent Publication No. 2005-205477A

PLT 4: Japanese Patent Publication No. 2007-247001A

PLT 5: Japanese Patent Publication No. 2007-182608A

SUMMARY OF INVENTION Technical Problem

The present invention, in view of the above situation, has as its objectthe provision of steel sheet for a hot stamped member which enables theproduction of a product of high strength steel sheet which has anexcellent fatigue characteristic of the same extent as high strengthsteel sheet which is produced by controlling the composition of thesteel sheet or method of production (“ordinary high strength steelsheet”) when producing a product by applying the hot stamping method tosteel sheet and of a method of production of the same.

Solution to Problem

The inventors engaged in intensive research to solve this problem. As aresult, they discovered that making the deviation in hardness near thesurface layer of steel sheet before hot stamping within a predeterminedrange is extremely effective for improving the fatigue characteristic ofthe steel sheet after hot stamping (product). They discovered that suchsteel sheet can be obtained by controlling the conditions whenrecrystallization-annealing the cold rolled steel sheet, conductedrepeated tests, and thereby completed the present invention.

The gist of the invention is as follows:

(1) Steel sheet for a hot stamped member which includes compositionwhich contains, by mass %,

C: 0.15 to 0.35%, Si: 0.01 to 1.0%, Mn: 0.3 to 2.3%, Al: 0.01 to 0.5%,and

a balance of Fe and unavoidable impurities, and limit the impurities toP: 0.03% or less,S: 0.02% or less, andN: 0.1% or less,wherein a standard deviation of Vicker's hardness at a position of 20 μmfrom the steel sheet surface in the sheet thickness direction is 20 orless.

(2) The steel sheet for a hot stamped member as set forth in (1) whichfurther contains, by mass %, one or more of elements selected from

Cr: 0.01 to 2.0%, Ti: 0.001 to 0.5%, Nb: 0.001 to 0.5% B: 0.0005 to0.01%, Mo: 0.01 to 1.0% W: 0.01 to 0.5%, V: 0.01 to 0.5%, Cu: 0.01 to1.0%, and Ni: 0.01 to 5.0%.

(3) The steel sheet for a hot stamped member as set forth in (1) or (2)which has on the surface of the steel sheet one of a 5 μm to 50 μm thickAl plating layer, a 5 μm to 30 μm thick galvanized layer, or a 5 μm to45 μm thick Zn—Fe alloy layer.

(4) A method of production of steel sheet for a hot stamped membercomprising recrystallization-annealing cold rolled steel sheet whichincludes composition which contains, by mass %,

C: 0.15 to 0.35%, Si: 0.01 to 1.0%, Mn: 0.3 to 2.3%, Al: 0.01 to 0.5%,and

a balance of Fe and unavoidable impurities, and limit the impurities toP: 0.03% or less,S: 0.02% or less, andN: 0.1% or less,in which step, includinga first stage of heating by an average heating rate of 8 to 25° C./secfrom room temperature to a temperature M (° C.) andthen a second stage of heating by an average heating rate of 1 to 7°C./sec to a temperature S (° C.),wherein the temperature M (° C.) is 600 to 700 (° C.) and thetemperature S (° C.) is 720 to 820 (° C.).

(5) The method of production of steel sheet for a hot stamped member asset forth in (4) wherein the steel further contains, by mass %, one ormore of

Cr: 0.01 to 2.0%, Ti: 0.001 to 0.5%, Nb: 0.001 to 0.5% B: 0.0005 to0.01%, Mo: 0.01 to 1.0% W: 0.01 to 0.5%, V: 0.01 to 0.5%, Cu: 0.01 to1.0%, and Ni: 0.01 to 5.0%.

(6) The method of production of steel sheet for a hot stamped member asset forth in (4) or (5) wherein a hot rolling rate in the hot rollingstep is 60 to 90%, while a cold rolling rate of the cold rolling step is30 to 90%.

(7) The method of production of steel sheet for a hot stamped member asset forth in any one of (4) to (6) which further includes, after therecrystallization-annealing step, a step of dipping the steel sheet inan Al bath to form an Al plating layer on the surface.

(8) The method of production of steel sheet for a hot stamped member asset forth in any one of (4) to (6) which further includes, after therecrystallization-annealing step, a step of dipping the steel sheet in agalvanization bath to form a galvanized layer on the surface.

(9) The method of production of steel sheet for a hot stamped member asset forth in any one of (4) to (6) which further includes, after therecrystallization-annealing step, a step of dipping the steel sheet in aZn bath to form a galvanized layer on the surface, then further heatingto 600° C. or less to form a Zn—Fe alloy layer on the surface.

Advantageous Effects of Invention

The steel sheet for a hot stamped member of the present invention can beproduced by a known steelmaking facility. Further, a shaped part whichis obtained using the steel sheet for a hot stamped member of thepresent invention for shaping by widespread hot stamping facilities (hotstamped members) has a fatigue characteristic equal to “ordinary highstrength steel sheet” of the same strength, so has the effect ofexpanding the scope of application of hot stamped members (parts).

BRIEF DESCRIPTION OF INVENTION

FIG. 1 is perspective view which shows a sheet press die for hotstamping which is used for the examples of the present invention.

FIG. 2 is a view which shows fatigue test pieces.

FIG. 3 is a perspective view which shows locations of measurement ofhardness in a test piece for hardness measurement use of the samedimensions as the crack growth region of the fatigue test piece which isshown in FIG. 2.

FIG. 4 is a graph which shows the correlation between the fatigue limitratio and standard deviation of hardness before hot stamping of steelsheet for a hot stamped member of Example 1.

FIG. 5 is a perspective view which schematically shows steel sheet(member) which is formed into a hat shape by the hot stamping method.

FIG. 6 is a graph which shows the correlation between the fatigue limitratio and standard deviation of hardness before hot stamping of steelsheet for a hot stamped member of Example 2.

DESCRIPTION OF EMBODIMENTS

The inventors engaged in research using steel sheet which contains, bymass %, C: 0.23%, Si: 0.5%, and Mn: 1.6% to prepare a hot stamped memberand evaluated its characteristics. They discovered that the fatiguecharacteristic is one of the same but that there are hot stamped memberswhich are the same in composition of the steel sheet and almost the samein tensile strength, but differ in fatigue characteristic. Therefore,they investigated the differences of these in detail, whereupon theylearned that there are differences in the deviation in hardness near thesurface layers of hot stamped members. Accordingly, they further changedthe composition and recrystallization conditions of cold rolled steelsheet over a broad range to investigate the fatigue characteristic ofhot stamped members and discovered that there is a strong correlationbetween the fatigue characteristic of hot stamped members and thedeviation in surface hardness of the same and that to obtain a hotstamped member which is excellent in fatigue characteristic, it iseffective to make the various in surface hardness of steel sheet beforehot stamping within a predetermined range and that further to obtainsuch steel sheet, it is possible to control the conditions whenrecrystallization-annealing cold rolled steel sheet to a predeterminedrange.

Details will be explained in the examples, but the inventors used thesetest findings as the basis to experimentally clarify the suitable rangeof deviation in hardness and the annealing conditions and therebycompleted the present invention.

Composition of Steel Sheet

First, the composition of steel sheet will be explained. Here, the “%”in the composition mean mass %.

C: 0.15 to 0.35%

C is the most important element in increasing the strength of steelsheet by hot stamping. To obtain a 1200 MPa or so strength after hotstamping, 0.15% or more has to be included. On the other hand, if over0.35% is included, deterioration of toughness is a concern, so 0.35% ismade the upper limit.

Si: 0.01 to 1.0%

Si is a solution strengthening element. Up to 1.0% can be effectivelyutilized. However, if more than that is included, trouble is liable tooccur at the time of chemical treatment or coating after shaping, so1.0% is made the upper limit. The lower limit is not particularlylimited. The effect of the present invention can be obtained. However,reduction more than necessary just raises the steelmaking load, so thecontent is made the level of inclusion due to deoxidation, that is,0.01% or more.

Mn: 0.3 to 2.3%

Mn is an element which functions as a solution strengthening element inthe same way as Si and also is effective for raising the hardenabilityof steel sheet. This effect is recognized at 0.3% or more. However, evenif over 2.3% is included, the effect becomes saturated, so 2.0% is madethe upper limit.

P: 0.03% or less, S: 0.02% or less

The two elements are both unavoidable impurities. They affect the hotworkability, so have to be limited to the above ranges.

Al: 0.01 to 0.5%

Al is suitable as a deoxidizing element, so 0.01% or more should beincluded. However, if included in a large amount, coarse oxides areformed and the mechanical properties of the steel sheet are impaired, sothe upper limit is made 0.5%.

N: 0.1% or less

N is an unavoidable impurity. It easily bonds with Ti or B, so has to becontrolled so as not to reduce the targeted effect of these elements.0.1% or less is allowable. The content is preferably 0.01% or less. Onthe other hand, reduction more than necessary places a massive load onthe production process, so 0.0010% should be made the target for thelower limit.

Cr: 0.01 to 2.0%

Cr has the effect of raising the hardenability, so can be suitably used.This effect becomes clear at 0.01% or more. On the other hand, even ifover 2.0% is added, this effect becomes saturated, so 2.0% is made theupper limit.

Ti: 0.001 to 0.5%

Ti is an element which acts to stably draw out the effect of B,explained later, through the formation of its nitride, so can beeffectively used. For this reason, 0.001% or more has to be added, butif excessively added, the nitrides become excessive and deterioration intoughness or shear surface properties is invited, so 0.5% is made theupper limit.

Nb: 0.001 to 0.5%

Nb is an element which forms carbonitrides and raises the strength, socan be effectively used. This effect is recognized at 0.001% or more,but if over 0.5% is included, the controllability of the hot rolling isliable to be impaired, so 0.5% is made the upper limit.

B: 0.0005 to 0.01%

B is an element which raises the hardenability. The effect becomes clearat 0.0005% or more. On the other hand, excessive addition leads todeterioration of hot workability and a drop in the ductility, so 0.01%is made the upper limit.

Mo: 0.01 to 1.0%, W: 0.01 to 0.5%, V: 0.01 to 0.5%

These elements all have the effect of raising the hardenability, so canbe suitably used. The effect becomes clear in each case at 0.01% ormore. On the other hand, it is an expensive element, so theconcentration where the effect becomes saturated is preferably made theupper limit. For Mo, this is 1.0%, while for W and V, it is 0.5%.

Cu: 0.01 to 1.0%

Cu has the effect of raising the strength of the steel sheet by additionof Cu in 0.01% or more. However, excessive addition detracts from thesurface quality of the hot rolled steel sheet, so 1.0% is made the upperlimit.

Ni: 0.01 to 5.0%

Ni is an element which has the effect of raising the hardenability, socan be effectively used. The effect becomes clear at 0.01% or more. Onthe other hand, it is an expensive element, so 5.0% where the effectbecomes saturated is made the upper limit. Further, it also acts tosuppress the drop in the surface quality of the hot rolled steel sheetdue to Cu, so inclusion simultaneously with Cu is desirable.

Note that in the present invention, the composition other than the aboveconsist of Fe, but unavoidable impurities which enter from the scrap andother melting materials or the refractories etc. are allowed.

Deviations in Steel Sheet Surface Hardness

The deviations in steel sheet surface hardness will be explained.

First, the method of determining (measuring) the hardness of the steelsheet surface will be explained.

The hardness of the steel sheet surface ideally should be measured by ahardness meter (for example Vicker's hardness meter) with the steelsheet surface facing upward and with the sheet thickness directionmatched with the vertical direction, but to clearly determineindentations (measure dimensions of indentations precisely), the surface(measurement surface) has to be polished or other certain work isnecessary. In such work (for example, mechanical polishing), at leastseveral dozen μm or so are removed from the original surface. Further,even if removing part of the surface using an acid etc. to chemicallypolish it, there is no difference. Rather, the smoothness is oftendegraded. Therefore, using such a technique to determine (measure) thehardness of the steel sheet surface is not practical.

Therefore, the inventors decided to determine the hardness at across-section parallel to the sheet thickness direction of the steelsheet. By doing so, the steel sheet surface can be measured withoutworking it (without removing the steel sheet surface). However, in thiscase as well, the position able to be measured by a hardness meter inthis way is inside from the surface a slight amount in the sheetthickness direction. For this reason, as a next best solution, theinventors attempted to obtain information on a portion close to thesurface by making an indentation by as low a load as possible.

Specifically, refer to FIG. 3. First, the measurement surface (steelsheet cross-section) was polished to a mirror finish. A Vicker'shardness meter was used with a test load (load pushing in indenter) of10 gf, a pushing time of 15 seconds, and a measurement position in thesheet thickness direction of 20 μm from the steel sheet surface. The“hardness of the steel sheet” as used in the Description indicates thehardness determined based on the above technique.

Further, the hardness of the steel sheet surface in steel sheet whichhas as a surface layer of the steel sheet either an Al plating layer,galvanized layer, and Zn—Fe alloy layer was measured at a position 20 μmfrom the boundary (interface) between the plating layer and the steelsheet.

For example, the Al plating layer of the steel sheet which is used inthe examples is deemed to be comprised of an outside layer which has Alas its main composition and an inside (steel sheet side) layer which isbelieved to be a reaction layer of Al and Fe, so the hardness wasmeasured at a position 20 μm from the boundary of the inside layer andthe steel sheet in the sheet thickness direction and this was used asthe surface hardness of the steel sheet.

Next, the galvanized layer of the steel sheet which is used in theexamples is deemed to be comprised of two layers of an outside layerwhich has Zn as its main composition and an inside layer which is areaction layer of Al which was added in a fine amount in the Zn bath andFe, so the hardness was measured at a position 20 μm from the boundaryof the inside layer and the steel sheet in the sheet thickness directionand this was used as the surface hardness of the steel sheet.

Further, the Zn—Fe alloy layer of the steel sheet which is used in theexamples is deemed to be comprised of a plurality of alloy layers whichare comprised of Zn and Fe, so the hardness was measured at a position20 μm from the boundary of the inside-most layer and the steel sheet inthe sheet thickness direction and this was used as the surface hardnessof the steel sheet.

For the purpose of finding the deviation in hardness, the abovemeasurement was performed in the region corresponding to the fatiguecrack growth region (21) of the fatigue test piece which is shown inFIG. 2. FIG. 3 is a perspective view which shows the location ofmeasurement of the hardness. The indenter of the Vicker's hardness meterwas pushed in at a position of 20 μm from the surface or the steel sheetor the interface of the steel sheet and the plating layer in the sheetthickness direction. This operation, as shown in FIG. 3, was performedat indentation intervals of 0.1 mm in a direction parallel to thesurface of the steel sheet at 300 points per measurement sample (over 30mm by measurement length) (first measurement surface). Further, the sameoperation was performed at another location 5 mm from the firstmeasurement surface taken in advance (second measurement surface).

The hardnesses were found for the total 600 points in this way. Thestandard deviation using this as the population was calculated and usedas an indicator of the deviation.

Note that the above measurement length of 30 mm and the two locations 5mm apart were determined so as to match with the crack growth region ofthe fatigue test piece which is explained later.

In the experiment which is explained in the examples, samples with afatigue limit ratio after hot stamping of 0.4 or more and ones with aratio below that were compared for deviation in hardness of the steelsheet surface, whereupon in the former, the standard deviation was 40 orless. Therefore, the inventors proceeded with more detailedinvestigations, whereupon it became clear that the deviation in hardnessafter hot stamping has a standard deviation of 40 or less when thedeviation in hardness of the steel sheet before hot stamping, determinedby a similar technique, has a standard deviation of 20 or less.

In the present invention, the standard deviation of the Vicker'shardness at a position 20 μm from the steel sheet surface in the sheetthickness direction was defined as 20 or less based on such experimentalfindings.

Method of Production of Steel Sheet for Hot Stamped Member

Finally, the method of production of steel sheet for a hot stampedmember of the present invention will be explained.

The steel sheet for a hot stamped member of the present invention isprocessed in the accordance with the usual methods by the steps ofsteelmaking, casting, hot rolling, pickling, and cold rolling to obtaincold rolled steel sheet. The composition is adjusted to theabove-mentioned scope of the present invention in the steelmaking step,the steel is cast to a slab in the continuous casting step, then theslab is started to be hot rolled at for example a 1300° C. or lessheating temperature. The rolling is ended around 900° C. The coilingtemperature can be selected as, for example 600° C. etc. The hot rollingrate may be made 60 to 90%. The cold rolling is performed after thepickling step. The rolling rate can be selected from 30 to 90% in range.

The annealing step for recrystallizing the cold rolled steel sheet whichwas produced in this way is extremely important. The annealing step isperformed using a continuous annealing facility and is comprised of twostages of a first step of heating by an average heating rate of 8 to 25°C./sec from room temperature to the temperature M (° C.) and a secondstage of then heating by an average heating rate of 1 to 7° C./sec downto a temperature S (° C.). Here, the temperature M has to be 600 to700(° C.), and the temperature S has to be 720 to 820(° C.). Theseconditions are determined based on the results of the experiment whichis explained in the examples which are described below.

The reason why, when recrystallization-annealing under these conditions,the standard deviation of the Vicker's hardness which was measured at aposition of 20 μm from the steel sheet surface in the sheet thicknessdirection is 20 or less, that is, steel sheet with a small deviation inhardness is obtained, is not necessarily clear, but the distribution ofcrystal grain size is preferably as uniform as possible and thedimensions and distribution of carbides are also preferably similarly asuniform as possible, so the following may be guessed from the viewpointof the distribution of recrystallized particle size and the dimensionsand distribution of carbides.

The recrystallization process of cold rolled steel sheet is complicated,so it is not suitable to separate and independently discuss the meaningsof the heating rate for the phenomenon called recrystallization and thehighest heating temperature at that heating rate.

Therefore, first, regarding the first stage, for example, consider thecase where the heating rate is small and where it is large with respectto a certain single temperature M (° C.). It is believed that in theformer case, that is, when the heating rate is small, the density ofrecrystallization nuclei is (relatively) low and the individualrecrystallized grains freely grow, but in the high temperature regionnear M (° C.), fine recrystallized grains are produced from theremaining non-recrystallization region and, at the stage where thetemperature of the steel sheet reaches M (° C.), (relatively) largecrystal grains and small crystal grains are mixed.

On the other hand, it is believed that in the case of the latter, thatis, when the heating rate is large, the density of recrystallized grainnuclei is high, a large number of recrystallized grains grow at a fastrate, and the grain boundaries become closer and further, in the hightemperature region near M (° C.), the recrystallized grains compete ingrowth and as a result crystal grains which have specific crystalorientations grow while eating away at crystal grains which have othercrystal orientations, so at the stage when reaching M (° C.), it isbelieved there are large crystal grains and small crystal grains mixedtogether. Therefore, a combination of the suitable heating rate and M (°C.) whereby the recrystallized grains become close in grain boundariesat the stage where the temperature reaches M (° C.) becomes necessaryfor achieving a more uniform distribution of recrystallized particlesizes. The 8 to 25° C./sec of the average heating rate of the firststage and the 600 to 700° C. of the temperature M (° C.) are believed tocorrespond to these suitable conditions.

Next, to control competition of growth of recrystallized grains afterthe temperature of the steel sheet reaches M (° C.), the heating rate ofthe second stage has to be made smaller than the first stage. Further,in the temperature region from the temperature M (° C.) to thetemperature S (° C.), reformation of carbides due to the diffusion ofcarbon becomes active, so the combination of the setting of the highesttemperature S (° C.) of the annealing step and the heating rate up tothat temperature has important meaning.

When the heating rate is small for one S (° C.), the carbides which werepresent at the temperature M (° C.) uniformly grow, so it may be that asteel sheet results in which carbides of various dimensions which werepresent in the stage when reaching the temperature M (° C.) are presentin various ways. On the other hand, when the heating rate is large,small carbides disappear and large carbides grow and therefore thedimensions of the carbides become closer to uniform ones relativelyspeaking, but the density becomes small. Therefore, unevenness ofhardness of the steel sheet is caused due to the carbides. As opposed tothese, when the combination of the heating rate and the temperature S (°C.) of the second stage is suitable, the small carbides growpreferentially and it may be that a steel sheet results in whichrelatively uniform dimension carbides are dispersed at a suitabledensity, so the unevenness of hardness of the steel sheet due tocarbides becomes uneven. The 1 to 7° C./sec of the heating rate of thesecond stage and the 720 to 820° C. of the temperature S correspond tosuch suitable conditions.

After reaching the temperature S, the temperature S may be held for ashort time or the next cooling step may be immediately shifted to. Whenholding the temperature S, from the viewpoint of coarsening of thecrystal grains, the holding time is preferably 180 seconds or less, morepreferably 120 seconds or less.

The cooling rate from the temperature S in the cooling step is notparticularly limited, but 30° C./sec or more rapid cooling is preferablyavoided. Therefore, the cooling rate from the temperature S is less than30° C./sec, preferably 20° C. or less, more preferably 10° C. or less.Steel sheet for hot stamping use is often sheared to a predeterminedshape and then used for hot stamping. This is because it is feared thatrapid cooling raises the shear load and lowers the productionefficiency.

After annealing, the sheet may be cooled down to room temperature.During cooling, it may be dipped in a hot dip Al bath to form an Alplating layer.

The hot dip Al bath may contain 0.1 to 20% of Si.

The Si which is contained in the Al plating layer affects the reactionof Al and Fe which occurs during heating before hot stamping. Excessivereaction is liable to detract from the press formability of the platinglayer itself. On the other hand, excessive control of the reaction isliable to invite adherence of Al on the press forming die. To avoid sucha problem, the content of Si in the Al plating layer is preferably 1 to15%, more preferably 3 to 12%.

Further, during the cooling after annealing, the sheet was dipped in ahot dip galvanization bath to form a galvanized layer.

Furthermore, the sheet was dipped in a hot dip galvanization bath toform a galvanized layer, then was heated to 600° C. or less to form aZn—Fe alloy layer.

The hot dip galvanization bath could contain 0.01 to 3% of Al.

The existence of Al has a strong affect on the reaction of Zn and Fe.When forming a galvanized layer, the reaction layer of the Fe and Albecomes an obstacle and suppresses mutual dispersion of Zn and Fe. Onthe other hand, a Zn—Fe alloy layer is comprised of a Zn-rich alloylayer (ζ-phase, δ₁-phase) and Fe-rich alloy layer (Γ₁-phase, Γ-phase),but the former is rich in adhesion with the base iron, but theworkability is degraded, while the latter is excellent in workability,but is insufficient in adhesion. Therefore, it is necessary to suitablycontrol the ratio of composition of these four phases to satisfy thetargeted properties (giving preference to adhesion, giving preference toworkability, or balancing the two etc.) This can be performed byincluding in the hot dip galvanization bath 0.01 to 3% of Al so as toenable control of the diffusion of Fe. What sort of concentration to usemay be selected by the manufacturer in accordance with the ability orobjective of the production facility.

The thicknesses of the Al plating layer, galvanized layer, and Zn—Fealloy layer do not influence the fatigue characteristic of the steelsheet after hot stamping or the fatigue characteristic of the parts, butif excessively thick, the press formability is liable to be affected. Asshown in the examples, when the thickness of the Al plating layer isover 50 μm, the phenomenon of galling is recognized. When the thicknessof the Zn plating layer exceeds 30 μm, adhesion of the Zn to the diefrequently occurs. When the thickness of the Zn—Fe alloy layer is over45 μm, scattered cracking of the alloy layer is seen, and theproductivity is otherwise impaired. Therefore, the thicknesses of thelayers are preferably made Al plating layer: 50 μm or less, galvanizedlayer: 30 μm or less, and Zn—Fe alloy layer: 45μm or less.

When these plating layers are thin, there is no problem at all inshapeability, but from the viewpoint of the corrosion resistance, whichis aimed at imparting these plating layers, the lower limits of theplating layers are preferably made as follows: That is, the limits arethe Al plating layer: preferably 5 μm or more, more preferably 10 μm ormore, the galvanized layer: preferably 5 μm or more, more preferably 10μm or more, and the Zn—Fe alloy layer: preferably 5 μm or more, morepreferably 10 μm or more.

EXAMPLES

Below, examples will be used as the basis to explain the presentinvention in detail.

Example 1

Steels “a” to “f” which have the composition which is shown in Table 1were produced and cast. The slabs were heated to 1250° C. and suppliedto a hot rolling step where they were hot rolled at a final temperatureof 900° C. and a coiling temperature of 600° C. to obtain thickness 3.2mm steel sheets. These hot rolled steel sheets were pickled, then coldrolled to obtain thickness 1.6 mm cold rolled steel sheets.

The cold rolled steel sheets were recrystallized and annealed under theconditions of i to xviii described in Table 2 to obtain the steel sheetsfor hot stamped members 1 to 32 which are shown in Table 3. From part,two test pieces for measurement of the hardness before hot stamping wereobtained. The positions for sampling the test pieces were made positions5 mm separated in the width direction of the obtained steel sheet forhot stamped member.

The average heating rate 1 (first stage) and average heating rate 2(second stage) in Table 2 respectively show the average heating ratesfrom room temperature to temperature M (° C.) and the average heatingrate from temperature M (° C.) to the temperature S (° C.).

These steel sheets for hot stamped members were held at 900° C. for 10minutes, then were sandwiched by the test-use sheet press die which wasshown in FIG. 1 and hot stamped. Each type of steel sheet for a hotstamped member was used hot stamping 10 pieces. From one among these,two tensile test pieces based on the provisions of JIS No. 5 and twotest pieces for measurement of hardness (same procedure as with hotstamping) were obtained. From the remaining nine, two fatigue testpieces which are shown in FIG. 2 each, for a total of 18, were obtained.The method of working for obtain test pieces was electrodischargemachining.

A tensile test was performed to find the tensile strength σ_(B) (averagevalue of two tensile test pieces). On the other hand, 18 test pieceswere used to run a plane bending fatigue test and determine the 1×10⁷cycle fatigue strength σ_(W). The test conditions were a stress ratio of−1 and a repetition rate of 5 Hz.

The test pieces for measurement of hardness were polished to a mirrorfinish at cross-sections parallel to the rolling directions of coldrolled steel sheets both before and after hot stamping.

The hardness at 20 μm inside from the surfaces of these test pieces inthe sheet thickness direction was measured using a Vicker's hardnessmeter (HM-2000 made by Mitsutoyo). The pushing load was made 10 gf, thepushing time was made 15 seconds, and the measurement interval in thedirection parallel to the surface made 0.1 mm for measurement of 300points.

Two test pieces were measured in the same way. The standard deviation ofhardness was calculated from the data of the Vicker's hardness of atotal of 600 points.

Table 3 shows the steel number, processing conditions, standarddeviation of hardness before hot stamping, tensile strength σ_(B)(average of two), strength σ_(W), fatigue limit ratio σ_(W)/σ_(B), andstandard of hardness after hot stamping. The correlation between thefatigue limit ratio σ_(W)/σ_(B) and the standard deviation of hardnessbefore hot stamping is shown in FIG. 4.

It was learned that the tensile strength σ_(B) of steel sheet after hotstamping is almost entirely unaffected by therecrystallization-annealing conditions in steel sheet of the samecomposition (code “b”). On the other hand, the fatigue characteristics(σ_(W)/σ_(B)) were strongly affected by the recrystallization-annealingconditions.

In steel sheets using the annealing conditions iii, iv, vii, viii, xv,and xviii of the present invention, relatively high fatiguecharacteristics, that is, a 0.4 or more fatigue limit ratio(σ_(W)/σ_(B)), could be obtained in the range of about 1200 to 1500 MPain tensile strength. As opposed to this, in steel sheets which wereannealed under conditions outside the scope of the present invention,the obtained fatigue limit ratio was a low level of about 0.3.

This difference is due to the fact that the fatigue limit ratio iscorrelated with the standard deviation of hardness after hot stamping.Simultaneously, it clearly depends on the standard deviation of thehardness before hot stamping. As shown in Nos. 1 to 6, 8, 9, 12, 13, 16,17, 20, 21, and 23 to 28, it became clear that when the standarddeviation of the hardness is 2 or less, a hot stamped member which hasan excellent fatigue characteristic (high fatigue limit ratio) isobtained.

Further, as the conditions of recrystallization-annealing for obtainingsteel sheet with a standard deviation of hardness before hot stamping of20 or less, there are a first stage of heating by an average heatingrate of 15 to 25° C./sec from room temperature to a temperature M (° C.)and a second stage of then heating by an average heating rate of 2 to 5°C./sec to the temperature S (° C.). It became clear that M is 620 to 680(° C.) and S is 780 to 820(° C.).

TABLE 1 Steel no. C Si Mn P S Al N Others a 0.25 0.7 1.9 0.02 0.002 0.030.004 Ti: 0.03, B: 0.003 b 0.23 0.5 1.6 0.02 0.002 0.03 0.003 c 0.21 0.31.4 0.02 0.002 0.03 0.002 B: 0.004 d 0.20 0.2 1.2 0.02 0.002 0.03 0.004Cr: 0.2, Ti: 0.02, B: 0.002 e 0.18 0.2 1.3 0.02 0.002 0.03 0.003 Cr:1.4, Ti: 0.02, B: 0.002 f 0.15 0.3 1.1 0.02 0.002 0.03 0.003 Cr: 0.1, B:0.004 Units are mass %.

TABLE 2 Average Average Condition heating rate Temp. M heating rateTemp. no. 1 (° C./sec) (° C.) 2 (° C./sec) S (° C.) Cooling conditions i20 650 3 800 No holding. Cooling Inv. ex. by average cooling rate 6°C./sec to 670° C., holding for 10 seconds, then air cooling to roomtemperature. ii 25 590 3 800 No holding. Cooling Comp. ex. by averagecooling rate 6° C./sec to 670° C., holding for 10 seconds, then aircooling to room temperature. iii 25 600 3 800 No holding. Cooling Inv.ex. by average cooling rate 6° C./sec to 670° C., holding for 10seconds, then air cooling to room temperature. iv  8 700 3 800 Noholding. Cooling Inv. ex. by average cooling rate 6° C./sec to 670° C.,holding for 10 seconds, then air cooling to room temperature. v  8 710 3800 No holding. Cooling Comp. ex. by average cooling rate 6° C./sec to670° C., holding for 10 seconds, then air cooling to room temperature.vi 15 650 7 830 No holding. Cooling Comp. ex. by average cooling rate 6°C./sec to 670° C., holding for 10 seconds, then air cooling to roomtemperature. vii 15 650 7 820 No holding. Cooling Inv. ex. by averagecooling rate 6° C./sec to 670° C., holding for 10 seconds, then aircooling to room temperature. viii 15 650 2 720 No holding. Cooling Inv.ex. by average cooling rate 6° C./sec to 670° C., holding for 10seconds, then air cooling to room temperature. ix 15 650 2 710 Noholding. Cooling Comp. ex. by average cooling rate 6° C./sec to 670° C.,holding for 10 seconds, then air cooling to room temperature. x  7 600 4800 No holding. Cooling Comp. ex. by average cooling rate 6° C./sec to670° C., holding for 10 seconds, then air cooling to room temperature.xi  8 600 4 800 No holding. Cooling Inv. ex. by average cooling rate 6°C./sec to 670° C., holding for 10 seconds, then air cooling to roomtemperature. xii 25 700 3 800 No holding. Cooling Inv. ex. by averagecooling rate 6° C./sec to 670° C., holding for 10 seconds, then aircooling to room temperature. xiii 26 700 3 800 No holding. Cooling Comp.ex. by average cooling rate 6° C./sec to 670° C., holding for 10seconds, then air cooling to room temperature. xiv 20 650   0.5 720 Noholding. Cooling Comp. ex. by average cooling rate 6° C./sec to 670° C.,holding for 10 seconds, then air cooling to room temperature. xv 20 6501 720 No holding. Cooling Inv. ex. by average cooling rate 6° C./sec to670° C., holding for 10 seconds, then air cooling to room temperature.xvi 20 650 7 820 No holding. Cooling Inv. ex. by average cooling rate 6°C./sec to 670° C., holding for 10 seconds, then air cooling to roomtemperature. xvii 20 650 8 820 No holding. Cooling Comp. ex. by averagecooling rate 6° C./sec to 670° C., holding for 10 seconds, then aircooling to room temperature. xviii 20 650 3 800 Holding for 10 Inv. ex.sec., then air cooling to room temperature Underlined FIGURES indicateoutside scope of present invention.

TABLE 3 Standard Standard deviation of σ_(W)/σ_(B) deviation of hardness(fatigue hardness Steel Processing before hot σ_(B) σ_(W) limit afterhot No. no. conditions stamping (MPa) (MPa) ratio) stamping 1 a i 101510 619 0.41 27 Inv. ex. 2 b i  9 1508 603 0.40 22 Inv. ex. 3 c i  61501 630 0.42 20 Inv. ex. 4 d i  8 1498 614 0.41 21 Inv. ex. 5 e i 111503 646 0.43 27 Inv. ex. 6 f i  7 1422 597 0.42 24 Inv. ex. 7 b ii 301512 484 0.32 46 Comp. ex. 8 b iii 12 1506 602 0.40 20 Inv. ex. 9 b iv16 1489 610 0.41 23 Inv. ex. 10 b v 29 1502 451 0.30 42 Comp. ex. 11 bvi 24 1499 465 0.31 44 Comp. ex. 12 b vii 13 1505 647 0.43 19 Inv. ex.13 b viii 11 1516 637 0.42 22 Inv. ex. 14 b ix 24 1511 453 0.30 43 Comp.ex. 15 b x 32 1522 502 0.33 51 Comp. ex. 16 b xi 16 1518 638 0.42 24Inv. ex. 17 b xii 19 1512 650 0.43 26 Inv. ex. 18 b xiii 33 1507 4520.30 49 Comp. ex. 19 b xiv 29 1500 480 0.32 46 Comp. ex. 20 b xv 12 1496598 0.40 22 Inv. ex. 21 b xvi 11 1506 617 0.41 25 Inv. ex. 22 b xvii 271503 496 0.33 45 Comp. ex. 23 a xviii 10 1510 634 0.42 19 Inv. ex. 24 bxviii  6 1512 605 0.40 12 Inv. ex. 25 c xviii  8 1503 601 0.40 14 Inv.ex. 26 d xviii 13 1509 649 0.43 24 Inv. ex. 27 e xviii 18 1499 600 0.4027 Inv. ex. 28 f xviii 11 1418 610 0.43 22 Inv. ex. Underlined FIGURESindicate outside scope of present invention.

Example 2

Steels 2a to 2h which have the composition which is shown in Table 4were produced and cast. The slabs were hot rolled under the sameconditions as Example 1 to obtain thickness 3.0 mm steel sheets. Thesehot rolled steel sheets were pickled, then cold rolled to 1.2 mm.

These steel sheets were recrystallized and annealed under conditions ofi, ix, and xviii of Table 2 to obtain steel sheets for hot stampedmembers.

From these steel sheets, test pieces for measurement of hardness wereobtained by the same procedure was in Example 1.

These steel sheets for a hot stamped member were held at 900° C. for 5minutes, then were formed to hat shapes which are shown in FIG. 5 by thehot stamping method. As shown in this figure, fatigue test pieces whichare shown in FIG. 2 and JIS No. 5 tensile test pieces were obtained fromthe top parts of the hats.

These test pieces were used by the same procedure as in Example 1 tofind the standard deviation of hardness before hot stamping and thetensile strength σ_(B) (average of two) and 1×10⁷ cycle fatigue strengthσ_(W) of the steel sheet after hot stamping (member).

Table 5 should these results. The correlation between the fatigue limitratio σ_(W)/σ_(B) and the standard deviation of the hardness before hotstamping is shown in FIG. 6.

In steel sheets for a hot stamped member which were recrystallized andannealed using conditions i and xviii in the scope of the presentinvention, even if steel sheets which contain Mo, W, V, Cu, and Ni, thedeviation in hardness of the surface layer before hot stamping had astandard deviation of 20 or less. Further, if using these, it becameclear that a hot stamped member with a fatigue limit ratio of 0.4 ormore, that is, excellent in fatigue characteristic, was obtained.

On the other hand, in steel sheets which were recrystallized andannealed using the condition ix which is outside the scope of thepresent invention, the deviation in hardness of the surface layer beforehot stamping has a standard deviation of over 20. The fatigue limitratio of the hot stamped members obtained by using these was 0.26 to0.31. It became clear the fatigue characteristic was inferior.

TABLE 4 Steel Composition (mass %) no. C Si Mn P S Al N Others 2a 0.350.3 1.0 0.02 0.004 0.03 0.004 Cr: 0.2, Ti: 0.01, B: 0.002, Cu: 0.1, Ni:0.1 2b 0.31 0.5 1.2 0.02 0.004 0.03 0.004 Cr: 0.5, Ti: 0.02, B: 0.004,Nb: 0.02, Mo: 0.2 2C 0.28 1.0 1.7 0.02 0.004 0.03 0.004 W: 0.2, Ni: 2.02d 0.25 0.8 1.9 0.02 0.004 0.03 0.004 Ti: 0.03, B: 0.003, Mo: 0.2, Ni:1.0 2e 0.23 0.6 1.6 0.02 0.004 0.03 0.003 Mo: 0.1, W: 0.5, V: 0.5 2f0.21 0.4 1.4 0.02 0.004 0.03 0.002 B: 0.004, Mo: 0.1, V: 0.5 2g 0.20 0.31.2 0.02 0.004 0.03 0.004 Cr: 0.2, Ti: 0.02, Mo: 0.2, W: 0.4 2h 0.18 0.31.3 0.02 0.004 0.03 0.003 Cr: 1.4, Ti: 0.02, B: 0.002, Mo: 0.1, V: 0.2

TABLE 5 Standard deviation of hardness σ_(W)/σ_(B) Steel Processingbefore hot σ_(B) (fatigue limit No. no. conditions stamping (MPa) σ_(W)(MPa) ratio) 29 2a i 18 1794 718 0.40 Inv. ex. 30 2a ix 40 1790 465 0.26Comp. ex. 31 2a xviii 19 1802 721 0.40 Inv. ex. 32 2b i 16 1706 682 0.40Inv. ex. 33 2b ix 37 1696 441 0.26 Comp. ex. 34 2b xviii 18 1711 7020.41 Inv. ex. 35 2C i 15 1598 639 0.40 Inv. ex. 36 2C ix 30 1592 4300.27 Comp. ex. 37 2C xviii 14 1590 636 0.40 Inv. ex. 38 2d i 15 1492 6120.41 Inv. ex. 39 2d ix 26 1500 435 0.29 Comp. ex. 40 2d xviii  5 1498614 0.41 Inv. ex. 41 2e i  9 1492 597 0.4 Inv. ex. 42 2e ix 31 1502 4210.28 Comp. ex. 43 2e xviii 10 1516 622 0.41 Inv. ex. 44 2f i 12 1508 6030.4 Inv. ex. 45 2f ix 36 1512 469 0.31 Comp. ex. 46 2f xviii 19 1522 6090.4 Inv. ex. 47 2g i 14 1496 613 0.41 Inv. ex. 48 2g ix 33 1504 406 0.27Comp. ex. 49 2g xviii 13 1526 641 0.42 Inv. ex. 50 2h i 14 1506 602 0.4Inv. ex. 51 2h ix 32 1512 454 0.3 Comp. ex. 52 2h xviii 15 1528 642 0.42Inv. ex. Underlined FIGURES indicate outside scope of present invention.

Example 3

Steels 3a to 3d which have the composition which is shown in Table 6were produced and cast. The slabs were hot rolled under the sameconditions as Example 1 to obtain thickness 2.5 mm steel sheets. Thesehot rolled steel sheets were pickled, then cold rolled to 1.2 mm.

These steel sheets were heated by an average heating rate of 19° C./secup to 655° C., then were heated by an average heating rate of 2.5° C. to800° C., then were immediately cooled by an average cooling rate of 6.5°C./sec. Further, they were dipped in a 670° C. hot dip Al bath(containing 10% of Si and unavoidable impurities), taken out after 5seconds, adjusted in amount of deposition by a gas wiper, then aircooled down to room temperature.

From the obtained steel sheets, the same procedure as in Example 1 wasused to obtain test pieces for measurement of hardness. To measure thehardness, the hardness at a position 20 μm from the boundary of theinside layer of the Al plating layer (reaction layer of Al and Fe) andthe steel sheet was measured by the same procedure as in Example 1. Atthe time of this measurement, the thickness of the Al plating layer(total of two layers) was also measured. The range of measurement ofthickness was made the same length 30 mm as the range of measurement ofhardness. Seven points were measured at measurement intervals of 5 mm ateach of the first measurement surface and second measurement surface fora total of 14 measurement positions. The average value was found.

These steel sheets were hot stamped into hat shapes by the sameprocedure as in Example 2. The heating conditions were holding at 900°C. for 1 minute.

From the top parts of the hats, fatigue test pieces which are shown inFIG. 2 and JIS No. 5 tensile test pieces were obtained.

These test pieces were used to find the tensile strength σ_(B) (averageof two) and 1×10⁷ cycle fatigue strength σ_(W). Table 7 shows theresults.

In all examples, excellent steel sheet for a hot stamped member with afatigue limit ratio of 0.4 or more was obtained, but in Nos. 57, 62, 67,and 72 where the thickness of the Al plating layer exceeded 50 μm, agalling phenomenon occurred at a high frequency at the long wall partsof the hat shape. In examples of 50 μm or less, no galling phenomenonoccurred at all. Therefore, it was judged that the upper limit ofthickness when Al plating the steel sheet surface is 50 μm or less.

TABLE 6 Steel no. C Si Mn P S Al N Others 3a 0.33 0.09 1.8 0.01 0.0040.04 0.003 Cr: 0.2, Mo: 0.2, Cu: 0.1, Ni: 0.05 3b 0.25 0.18 1.4 0.010.004 0.04 0.003 Cr: 0.002, Ti: 0.02. B: 0.003, Mo: 0.2, W: 0.1, V: 0.13C 0.22 0.12 1.3 0.02 0.008 0.03 0.004 Cr: 0.13, Ti: 0.03, Nb: 0.02, B:0.002 3d 0.15 0.33 1.0 0.02 0.008 0.03 0.004 B: 0.0005 Units are mass %.

TABLE 7 Standard deviation σ_(W)/σ_(B) of hardness (fatigue Thickness ofSteel before hot σ_(B) σ_(W) limit Al plating No. no. stamping (MPa)(MPa) ratio) layer (μm) 53 3a 17 1784 714 0.40 16.0 Inv. ex. 54 3a 181789 716 0.40 22.2 Inv. ex. 55 3a 16 1801 720 0.40 33.9 Inv. ex. 56 3a14 1792 717 0.40 48.6 Inv. ex. 57 3a 14 1790 716 0.40 51.0 Comp. ex. 583b 12 1516 652 0.43 15.1 Inv. ex. 59 3b 15 1520 638 0.42 19.6 Inv. ex.60 3b 19 1524 671 0.44 34.2 Inv. ex. 61 3b 18 1522 685 0.45 49.6 Inv.ex. 62 3b 20 1534 614 0.40 54.7 Comp. ex. 63 3C 11 1502 631 0.42 14.5Inv. ex. 64 3C 14 1509 649 0.43 20.1 Inv. ex. 65 3C 9 1513 635 0.42 34.6Inv. ex. 66 3C 13 1519 668 0.44 49.2 Inv. ex. 67 3C 18 1524 610 0.4055.3 Comp. ex. 68 3d 10 1318 554 0.42 17.2 Inv. ex. 69 3d 10 1326 5570.42 20.4 Inv. ex. 70 3d 8 1320 554 0.42 30.2 Inv. ex. 71 3d 14 1314 5390.41 42.0 Inv. ex. 72 3d 15 1310 537 0.41 53.6 Comp. ex. UnderlinedFIGURES indicate outside scope of present invention.

Example 4

Steels 3a to 3d which have the composition which is shown in Table 6were produced and cast. The slabs were hot rolled under the sameconditions as Example 1 to obtain thickness 2.5 mm steel sheets. Thesehot rolled steel sheets were pickled, then cold rolled to 1.2 mm.

These steel sheets were heated by an average heating rate of 19° C./secup to 655° C., then were heated by an average heating rate of 2.5° C. to800° C., then were immediately cooled by an average cooling rate of 6.5°C./sec. Further, they were dipped in a 460° C. hot dip galvanizationbath (containing 0.15% of Al and unavoidable impurities), taken outafter 3 seconds, adjusted in amount of deposition by a gas wiper, thenair cooled down to room temperature.

From the obtained steel sheets, the same procedure as in Example 1 wasused to obtain test pieces for measurement of hardness. To measure thehardness, the hardness at a position 20 μm from the boundary of theinside layer of the Zn plating layer (reaction layer of Al and Fe) andthe steel sheet was measured by the same procedure as in Example 1. Atthe time of this measurement, the thickness of only the Zn plating layermay also be measured. The range of measurement of thickness was made thesame length 30 mm as the range of measurement of hardness. Seven pointswere measured at measurement intervals of 5 mm at each of the firstmeasurement surface and second measurement surface for a total of 14measurement positions. The average value was found.

These steel sheets were hot stamped into hat shapes by the sameprocedure as in Example 2. They were heated to 880° C. and held for 5seconds, then air-cooled down to 700° C. and pressed.

From the top parts of the hats, fatigue test pieces which are shown inFIG. 2 and JIS No. 5 tensile test pieces were obtained.

These test pieces were used to find the tensile strength σ_(B) (averageof two) and 1×10⁷ cycle fatigue strength σ_(W). Table 8 shows theresults.

In all examples, excellent steel sheet for a hot stamped member with afatigue limit ratio of 0.4 or more was obtained, but in Nos. 77, 82, 87,and 92 where the thickness of the galvanized layer exceeded 30 μm,adhesion of Zn was observed at a high frequency in the die. In examplesof 30 μm or less, no adhesion of Zn occurred at all. Therefore, it wasjudged that the upper limit of thickness when galvanizing the steelsheet surface is 30 μm or less.

TABLE 8 Stan- dard devia- tion of hard- ness before σ_(W)/σ_(B)Thickness hot (fatigue of Steel stamp- σ_(B) σ_(W) limit galvanized No.no. ing (MPa) (MPa) ratio) layer (μm) 73 3a 17 1785 714 0.40  6.1 Inv.ex. 74 3a 17 1788 715 0.40 12.5 Inv. ex. 75 3a 16 1802 721 0.40 23.8Inv. ex. 76 3a 13 1794 718 0.40 28.6 Inv. ex. 77 3a 15 1793 717 0.4031.0 Comp. ex. 78 3b 12 1516 652 0.43 11.1 Inv. ex. 79 3b 15 1522 6390.42 19.6 Inv. ex. 80 3b 19 1534 675 0.44 24.8 Inv. ex. 81 3b 18 1532689 0.45 29.0 Inv. ex. 82 3b 20 1545 618 0.40 33.7 Comp. ex. 83 3c 101518 638 0.42 10.3 Inv. ex. 84 3c 14 1536 660 0.43 17.2 Inv. ex. 85 3c 91524 640 0.42 19.6 Inv. ex. 86 3c 14 1539 677 0.44 29.3 Inv. ex. 87 3c18 1544 618 0.40 32.3 Comp. ex. 88 3d 10 1336 561 0.42 11.2 Inv. ex. 893d 12 1342 564 0.42 17.4 Inv. ex. 90 3d 8 1318 554 0.42 20.2 Inv. ex. 913d 13 1320 541 0.41 28.0 Inv. ex. 92 3d 15 1330 545 0.41 33.4 Comp. ex.Underlined FIGURES indicate outside scope of present invention.

Example 5

Steels 3a to 3d which have the composition which is shown in Table 6were produced and cast. The slabs were hot rolled under the sameconditions as Example 1 to obtain thickness 2.5 mm steel sheets. Thesehot rolled steel sheets were pickled, then cold rolled to 1.2 mm.

These steel sheets were heated by an average heating rate of 19° C./secup to 655° C., then were heated by an average heating rate of 2.5° C. to800° C., then were immediately cooled by an average cooling rate of 6.5°C./sec. Further, they were dipped in a 460° C. hot dip galvanizationbath (containing 0.13% of Al, 0.03% of Fe, and unavoidable impurities),taken out after 3 seconds, adjusted in amount of deposition by a gaswiper, then heated to 480° C. to form an Zn—Fe alloy layer, then aircooled down to room temperature.

From the obtained steel sheets, the same procedure as in Example 1 wasused to obtain test pieces for measurement of hardness. To measure thehardness, the hardness at a position 20 μm from the boundary of theinner-most layer of the Zn—Fe alloy layer (reaction layer of Zn and Fe)and the steel sheet was measured by the same procedure as in Example 1.At the time of this measurement, the total thickness of the Zn—Fe alloylayer (which was comprised of four layers) was also measured. At thetime of this measurement, the thickness of the Al plating layer (totalof two layers) was also measured. The range of measurement of thicknesswas made the same length 30 mm as the range of measurement of hardness.Seven points were measured at measurement intervals of 5 mm at each ofthe first measurement surface and second measurement surface for a totalof 14 measurement positions. The average value was found.

These steel sheets were hot stamped into hat shapes by the sameprocedure as in Example 2. They were heated to 880° C. and held for 5seconds, then air-cooled down to 700° C. and pressed.

From the top parts of the hats, fatigue test pieces which are shown inFIG. 2 and JIS No. 5 tensile test pieces were obtained.

These test pieces were used to find the tensile strength σ_(B) (averageof two) and 1×10⁷ cycle fatigue strength σ_(W). Table 9 shows theresults.

In all examples, excellent steel sheet for a hot stamped member with afatigue limit ratio of 0.4 or more was obtained, but in Nos. 97, 102,107, and 112 where the thickness of the Zn—Fe alloy layer exceeded 45μm, fine cracks occurred in the alloy layer after pressing. In examplesof 45 μm or less, no fine cracks formed at all. Therefore, it was judgedthat the upper limit of thickness when forming a Zn—Fe alloy layer onthe steel sheet surface is 45 μm or less.

TABLE 9 Standard deviation σ_(W)/σ_(B) of hardness (fatigue Thickness ofSteel before hot σ_(B) σ_(W) limit Zn—Fe alloy No. no. stamping (MPa)(MPa) ratio) layer (μm) 93 3a 17 1773 727 0.41 15.0 Inv. ex. 94 3a 161777 711 0.40 22.2 Inv. ex. 95 3a 17 1802 739 0.41 31.5 Inv. ex. 96 3a14 1786 714 0.40 39.9 Inv. ex. 97 3a 13 1772 709 0.40 46.0 Comp. ex. 983b 12 1505 632 0.42 15.7 Inv. ex. 99 3b 18 1519 638 0.42 21.6 Inv. ex.100 3b 19 1513 651 0.43 39.2 Inv. ex. 101 3b 18 1502 661 0.44 44.6 Inv.ex. 102 3b 14 1518 622 0.41 49.7 Comp. ex. 103 3C 11 1506 633 0.42 14.5Inv. ex. 104 3C 14 1503 646 0.43 20.8 Inv. ex. 105 3C 9 1500 645 0.4334.6 Inv. ex. 106 3C 12 1506 633 0.42 42.2 Inv. ex. 107 3C 19 1510 6190.41 45.3 Comp. ex. 108 3d 17 1307 523 0.40 15.2 Inv. ex. 109 3d 11 1313551 0.42 18.4 Inv. ex. 110 3d 8 1320 554 0.42 30.6 Inv. ex. 111 3d 141314 539 0.41 42.9 Inv. ex. 112 3d 15 1310 537 0.41 48.6 Comp. ex.Underlined FIGURES indicate outside scope of present invention.

REFERENCE SIGNS LIST

-   11 a top die    -   11 b bottom die-   12 steel sheet-   21 fatigue crack growth region-   51 test piece sampling position

1. Steel sheet for a hot stamped member which includes composition whichcontains, by mass %, C: 0.15 to 0.35%, Si: 0.01 to 1.0%, Mn: 0.3 to2.3%, Al: 0.01 to 0.5%, and a balance of Fe and unavoidable impurities,and limit the impurities to P: 0.03% or less, S: 0.02% or less, and N:0.1% or less, wherein a standard deviation of Vicker's hardness at aposition of 20 μm from the steel sheet surface in the sheet thicknessdirection is 20 or less.
 2. The steel sheet for a hot stamped member asset forth in claim 1 which further contains, by mass %, one or more ofelements selected from Cr: 0.01 to 2.0%, Ti: 0.001 to 0.5%, Nb: 0.001 to0.5% B: 0.0005 to 0.01%, Mo: 0.01 to 1.0% W: 0.01 to 0.5%, V: 0.01 to0.5%, Cu: 0.01 to 1.0%, and Ni: 0.01 to 5.0%.
 3. The steel sheet for ahot stamped member as set forth in claim 1 which has on the surface ofsaid steel sheet one of a 5 μm to 50 μm thick Al plating layer, a 5 μmto 30 μm thick galvanized layer, or a 5 μm to 45 μm thick Zn—Fe alloylayer.
 4. A method of production of steel sheet for a hot stamped membercomprising recrystallization-annealing cold rolled steel sheet whichincludes composition which contains, by mass %, C: 0.15 to 0.35%, Si:0.01 to 1.0%, Mn: 0.3 to 2.3%, Al: 0.01 to 0.5%, and a balance of Fe andunavoidable impurities, and limit the impurities to P: 0.03% or less, S:0.02% or less, and N: 0.1% or less, in which step, including a firststage of heating by an average heating rate of 8 to 25° C./sec from roomtemperature to a temperature M (° C.) and then a second stage of heatingby an average heating rate of 1 to 7° C./sec to a temperature S (° C.),wherein the temperature M (° C.) is 600 to 700 (° C.) and thetemperature S (° C.) is 720 to 820 (° C.).
 5. The method of productionof steel sheet for a hot stamped member as set forth in claim 4 whereinsaid steel further contains, by mass %, one or more of Cr: 0.01 to 2.0%,Ti: 0.001 to 0.5%, Nb: 0.001 to 0.5% B: 0.0005 to 0.01%, Mo: 0.01 to1.0% W: 0.01 to 0.5%, V: 0.01 to 0.5%, Cu: 0.01 to 1.0%, and Ni: 0.01 to5.0%.
 6. The method of production of steel sheet for a hot stampedmember as set forth in claim 5 wherein a hot rolling rate in said hotrolling step is 60 to 90%, while a cold rolling rate of said coldrolling step is 30 to 90%.
 7. The method of production of steel sheetfor a hot stamped member as set forth in claim 4 which further includes,after said recrystallization-annealing step, a step of dipping saidsteel sheet in an Al bath to form an Al plating layer on the surface. 8.The method of production of steel sheet for a hot stamped member as setforth in claim 4 which further includes, after said recrystallizationannealing step, a step of dipping said steel sheet in a Zn bath to forma galvanized layer on the surface.
 9. The method of production of steelsheet for a hot stamped member as set forth in claim 4 which furtherincludes, after said recrystallization-annealing step, a step of dippingsaid steel sheet in a Zn bath to form a galvanized layer on the surface,then further heating to 600° C. or less to form a Zn—Fe alloy layer onsaid surface.